This invention relates to a liquid crystal display, and more particularly to a reduction in the electric power consumed by a back light thereof.
A liquid crystal display (LCD) is fabricated by enclosing liquid crystal between electrode substrates each of which is made by forming transparent electrodes on a transparent substrate. Liquid crystal has an electro-optical anisotropy. Therefore, when a predetermined voltage is applied between the electrodes such that an electric field is formed over the liquid crystal, the liquid crystal demonstrates optical characteristics, which depend on the strength of the electric field. A plurality of pixels are formed on the LCD. Accordingly, if a different voltage is applied to each pixel, a display image is formed as an aggregation of pixels having desired brightness. Thus, in the LCD, the display image is formed by voltage control. The LCD has certain advantages in that it is small, thin, and has low electric power consumption. Accordingly, it is desirable to use LCDS in office automation (OA) device and audio video (AV) device applications. LCDs are often used in portable devices, which are frequently used out of doors. In such cases, an LCD has been developed which uses ambient light to allow the display image to be visualized. This use of ambient light significantly reduces the gross electric power required by the LCD.
FIG. 1 is a side cross-sectional view of a conventional LCD 100. The LCD 100 has an LCD panel 10, a light conducting plate 11 for supplying light to the panel 10, a light diffusing plate 12 for diffusing light in the plate 11 to irradiate a uniform plane of light on the panel 10 and a light reflecting plate 13 for reflecting light in the plate 11 randomly. The plates 11, 12 and 13 are disposed within a housing 40. Further, a light source 20 for transmitting light to the conducting plate 11 and a light collecting lens 30 for receiving ambient light, such as outdoor daylight, and providing it to the conducting plate 11 are accommodated in the housing 40. The light source 20 may be a fluorescent lamp having a reflector 21 disposed at the back thereof and the conducting plate 11 may be constructed of acrylic resin. Instead of using the diffusing plate 12 and the reflecting plate 13, diffusive processing and randomly reflective processing may be applied to the front surface and the back surface of the conducting plate 11, respectively, so that the conducting plate 11 can provide a diffusive function and a reflective function. A back light portion or an illumination portion is formed by the conducting plate 11, the diffusing plate 12, the reflecting plate 13, light source 20 and the lens 30. The lens 30 may be mounted to the conducting plate 11 or integral with the conducting plate 11. The housing 40 has an opening 41 for exposing the lens 30 to ambient light.
Light emitted from the light source 20 or light received through the lens 30 is passed through the conducting plate 11 and diffused by the diffusing plate 12, and then randomly reflected by the reflecting plate 13. The light is then irradiated on the LCD panel 10 from the diffusing plate 12 as a uniform plane light. The LCD panel 10 cannot emit light by itself. Thus, the LCD panel 10 is illuminated from behind. The light permeability of the LCD panel 10 is controlled in order to distribute the light passing through the LCD panel 10 in a desired pattern to allow images to be formed on the LCD panel 10.
Under an environment where plenty of ambient light exists, such as on a sunny day, the LCD panel 10 is irradiated by light coming only from the lens 30 without requiring light from the light source 20. Conversely, when there is insufficient ambient light available, such as in the interior of a house, the LCD panel 10 is irradiated by light from the light source 20. Accordingly, gross electric power is reduced when the use of the light source 20 is not required, resulting in only using drive power for the LCD panel 10.
However, light introduced from the collecting lens 30 and the light source 20 runs principally along the plane of the LCD panel 10. Therefore, a portion of the light from the light source 20 passes through the inside of the conducting plate 11 parallel to the plane of the panel 10 and leaks out of the LCD 100 from the lens 30. A portion of light received from the lens 30 to the conducting plate 11 also runs parallel to the plane of the panel 10 through the inside of the conducting plate 11, reflects at the reflector 21, and then runs back through the conducting plate 11, and again leaks out of the LCD 100 from the lens 30. Thus, the amount of light captured by the diffusing plate 12 and the reflecting plate 13 and irradiated toward the LCD panel 10 is reduced, which reduces the efficiency of the panel 10.
Even if the light source 20 and the collecting lens 30 are positioned at adjacent sides, for example, light is still leaked from the lens 30. Thus, as long as the light source 20 and the collecting lens 30 are positioned in the same plane, a portion of the light transmitted through the conducting plate 11 is leaked out through the lens 30.
Further, the light source 20 is disposed within the LCD body and is not removable. Accordingly, even when use of the light source 20 is not required, the LCD (10 must be always carried with the light source 20, making the LCD unnecessarily heavy.
FIG. 2 is a side cross-sectional view of a conventional reflective type LCD 200. A reflective type LCD panel 110 is enclosed within a housing 140. The reflective type LCD panel 110 may employ reflective type electrodes, which reflect ambient light. The reflective type electrodes are arranged at the inner side of the LCD panel. Alternatively, LCD panel 110 may employ transparent electrodes and a reflecting plate, which is provided at the inner side of the LCD panel to reflect ambient light. The reflective type LCD panel 110 receives light from the front, observer side, modulates it by liquid crystal, reflects it at the reflecting plate and radiates it back to the observer side. Accordingly, the LCD 200 does not require an internal or artificial light source, which results in consumption of only power for driving the LCD panel 110, thus allowing for an overall reduction of power consumption.
However, although the LCD 200 operates well in an environment where a plenty of ambient light exists, the display screen is dark and invisible when there is insufficient ambient light.